Vapor compression systems, such as heat pumps, refrigeration and air-conditioning systems, are widely used in industrial and residential applications. The vapor compression systems are large consumers of electrical energy and require efficient means cooling and heating. It is desirable to improve the operating efficiency of the vapor compression cycle without increasing costs of the components of the vapor compression systems.
The introduction of variable speed compressors, variable position valves, and variable speed fans to the vapor compression cycle has greatly improved the flexibility of the operation. However, the increased flexibility of these variable-actuators requires careful control of the flow rate of the refrigerant mass. Cooling an environment (for example, a room in a house or a display case for food in a grocery store) is achieved by the evaporation of a refrigerant. The cooling includes evaporation process whereby a substance is converted from a liquid to a vapor. This process occurs as heat is absorbed by the refrigerant, thereby removing the heat from the space to be cooled. During the process when some refrigerant is in the liquid phase and some is in the vapor phase, the refrigerant is said to be a two-phase mixture.
This evaporation process occurs in a heat exchanger, commonly called an evaporator. For the most energy efficient operation, the amount of refrigerant that enters the evaporator should be carefully controlled. Ideally, the refrigerant, having a two-phase mixture of part liquid and part vapor, enters the evaporator, and through the process of evaporation, is entirely converted to vapor as the refrigerant exits the evaporator. If too little refrigerant enters the evaporator, then all of the refrigerant is prematurely converted to the vapor phase before the refrigerant exits the evaporator, which implies that a substantial fraction of an evaporator surface area was not used to perform the cooling, thereby reducing the efficiency of the system. Conversely, if too much refrigerant is allowed to enter the evaporator, the refrigerant exits the evaporator while still in the partly liquid phase state, which implies that the full potential to perform useful cooling is not performed in the evaporator, to also reduce the efficiency. In addition, the liquid components of refrigerant exiting the evaporator can be ingested into the compressor, which could potentially cause damage.
The evaporation process is generally a constant-temperature process (with the evaporating temperature depending on the material properties and pressure of the refrigerant during evaporation). After full evaporation, additional heat transferred to the vapor-phase refrigerant causes an increase in the temperature of the refrigerant temperature. The difference in temperature between the elevated vapor phase temperature and the evaporating temperature is called a superheat temperature. Refrigerant exiting an evaporator that has been completely evaporated into the vapor phase exhibits a temperature greater than the evaporating temperature, in other words, have a positive superheat temperature. Properly controlling the refrigerant superheat temperature at the evaporator exit optimizes efficiency, and protects the vapor compression equipment.
To directly measure refrigerant superheat, various methods determine the evaporating temperature at the inlet to the evaporator, which requires measuring the evaporating pressure. After the evaporating pressure is measured, the evaporating temperature can be calculated using refrigerant properties. The vapor temperature at the evaporator exit is also measured, and the superheat is computed as a difference in the measured outlet vapor temperature and the evaporating temperature. This direct measurement methods require at least one pressure sensor and one (more commonly two) temperature measurements. Sensors for measuring refrigerant pressure are costly and often unreliable, and therefore the direct measurement of superheat is usually limited to very expensive systems that can tolerate high component costs and provide redundant means of sensing.
Rather than directly measure superheat, it is possible to estimate the superheat using less expensive temperature sensors. For example U.S. Pat. No. 6,769,264 describes a method for estimating superheat with two dedicated temperature sensors arranged near the inlet and outlet of the evaporator heat exchanger. While this method may estimate the superheat temperature with reasonable accuracy, the method still requires additional dedicated temperature sensors located in particular positions on the heat exchanger. The additional sensors increase the cost of the machine.
U.S. Pat. No. 5,311,748 teaches that refrigerant may be controlled in such a way as to control superheat by using a combination of the compressor discharge temperature (temperature of the refrigerant exiting the compressor) and the outdoor air temperature. The valve position that controls the correct amount of refrigerant is computed using these sensors. While that method avoids unnecessary heat exchanger temperature or pressure sensors, the method is not suitable for vapor compression systems with variable speed compressors.
In consideration of the above, there is a need in the art for a method of measuring the amount of refrigerant entering the evaporator to optimize the efficiency and the cost of the vapor compression systems with variable speed compressors.